In a high power and high frequency device using a compound semiconductor device, in particular, a high electron mobility transistor (HEMT) containing nitride such as GaN, an increase of on-resistance caused by a current collapse is acknowledged as a problem. The current collapse occurs caused by an electric field concentration in a vicinity of a drain side edge of a gate electrode and so on. Accordingly, a structure in which a conductive film called as a field plate is connected to a source electrode has been studied to reduce the increase of the on-resistance caused by the current collapse and further, to reduce a lowering of an output current. The field plate is sometimes called as a source wall. In this structure, the field plate passes at upward of the gate electrode from the source electrode, and extents to an upward of an arbitrary position between the gate electrode and the drain electrode, and a ground potential is applied to the source electrode and the field plate. According to the structure, the electric field concentration between the gate electrode and the drain electrode is controlled, and the electric field intensity at the drain side edge of the gate electrode is lowered. The current collapse is thereby suppressed. In particular, it is effective for a high frequency and high power device used for a communication and so on.
However, the current collapse is reduced but on the other hand, a gain is also lowered in a conventional HEMT using a field plate.    [Patent Literature 1] Japanese Laid-open Patent Publication No. 2001-60684